Dual diaphragm differential pressure transducer

ABSTRACT

A corrosion proof differential pressure transducer includes a central disk and two thin ceramic diaphragms mounted close to but spaced from the disk on both sides thereof, with the diaphragms being intercoupled to move together. This may be accomplished, for example, by a rod passing through a hole in the center of the central disk which is secured to each of the diaphragms; or by a fluid such as silicone oil between the diaphragms and flowing through one or more holes through the central disk. The diaphragm, and disks and the rod are formed of alumina, quartz, or other inert, insulating material having substantially zero mechanical hysteresis. Conductive plates providing variable capacitance with changes in pressure are mounted on the inner surface one or both of the diaphragms and on the facing surface of the central disk. A housing and associated gaskets direct pressures from two different fluids to the two diaphragms, so that the differential pressure is measured.

FIELD OF THE INVENTION

This invention relates to differential pressure transducers.

BACKGROUND OF THE INVENTION

Capacitive pressure transducers are well known, and normally involve theuse of a diaphragm which is either formed of conductive material or hasa layer of conductive material on it, and as the diaphragm is shifted inposition with changes in pressure, the capacitance between theconductive layer and an adjacent capacitive plate is varied. One widelyused capacitive transducer is shown in U.S. Pat. No. 4,388,668, grantedJune 14, 1983, and assigned to the assignee of the present invention.

Differential pressure transducers are also known, and one suchtransducer is shown in U.S. Pat. No. 4,425,799, granted Jan. 17, 1984and also assigned to the assignee of the present invention. Adifferential pressure transducer measures the difference in pressurebetween two separate bodies of fluid, and in the case of the transducerdisclosed in U.S. Pat. No. 4,425,799, this is accomplished by applyingpressure to both sides of a diaphragm, with spaced conductive platesbeing secured to the diaphragm and to an adjacent fixed plate.Accordingly, as the diaphragm changes position under the opposing forcesfrom the two sources of pressure, the capacitance between the two platesvaries, and a resultant electrical signal varies to indicate thedifferential pressure.

The diaphragm may be made of an inert insulating material, such asalumina, which also has a low mechanical hysteresis. In the patentscited hereinabove, the use of either quartz, alumina, or glass such asPyrex, as the diaphragm and associated fixed plate, are disclosed.Electrically conductive layers are formed on the opposing surfaces ofthe spaced diaphragm and thicker plate, to provide the capacitanceplates of the variable capacitance. When this type of unit is employedas a differential transducer, the fluids to be measured may be appliedto both sides of the diaphragm. When this is done, the fluids to bemeasured, which may be corrosive, would contact the conductive metalplates and could corrode them. When the electrical plates are corroded,they may engage one another, or may change their output capacitance,both of which would make the transducer unreliable.

Accordingly, an important object of the present invention is to providea simple and reliable differential pressure transducer, and one which isnot adversely affected by corrosive fluids, the differential pressure ofwhich is being measured.

SUMMARY OF THE INVENTION

In accordance with the present invention, a differential pressuretransducer includes a central base plate made of inert insulatingmaterial, and having a central hole therethrough, and two diaphragms,made of thin insulating material having low mechanical hysteresismounted one on each side of the base member and spaced apart from thebase member by a distance of from about one-half a thousandth of an inchto about ten thousandths of an inch, and having the centers of the twodiaphragms coupled together. This coupling may be accomplished by a rod,extending through the central opening in the base plate and secured tothe centers of the two diaphragms, or alternatively by the use of afluid, such as silicone oil between the diaphragms, and flowing throughone or more holes in the central disk as the diaphragms flex. Thinlayers of conductive material are located on at least one of thediaphragms on the inner surface thereof, and on the facing surface ofthe base plate, so that when the two diaphragms flex together, thecapacitance between the plates will change. A housing and sealing orgasket arrangements are provided for applying pressure from two separatebodies or channels of fluid having different pressures, to the outersurfaces of the respective diaphragms.

The preferred embodiment of the invention may also include the followingadditional features:

1. The central base plate, the two diaphragms, and the interconnectingrod (when used) preferably have the same thermal coefficient ofexpansion, so that changes in temperature do not adversely affect theaccuracy of the unit.

2. The base plate, the diaphragms, and the interconnecting rod (whenused) may be formed of alumina, of quartz, of glass such a Pyrex glass,or other insulating material having substantially zero mechnicalhysteresis.

3. The base plate, and the two diaphragms are preferably circular, andhave flap opposed surfaces, and are between one and two inches indiameter.

4. Each of the diaphragms is preferably between about 0.010 inch andabout 0.25 inch in thickness, depending on the desired pressure range ofthe transducer.

5. The diaphragms are preferably sealed to the base plate and spacedfrom it around the peripheries of the diaphragms. The diaphragms may besecured to the base plate by glass frit, made up of two different typesof glass, which serves not only to space the diaphragm apart from thebase plate, by a predetermined distance determined by the particle sizeof the glass frit, and to seal the two elements together. Alternatively,the diaphragms may be secured to the base plate by other adhesives.

6. A plastic housing may be provided, with input ports or conduitsaxially aligned with the transducer, and with one input port and conduitfacing one of the diaphragms and the other input port facing the otherdiaphragm. A peripheral metal band with inturned edges may hold rubberrings around the periphery of the two diaphragms to seal them to theenclosing housing, thereby directing the input fluid pressure from oneport to the adjacent diaphragm, and similarly directing input pressurefrom the other input port or conduit to the other diaphragm.

When alumina or other inert materials are employed as the diaphragmmaterial, and the variable capacitance plates are in the form ofconductive layers on the inside of at least one of the diaphragms and onthe base plate, these conductive plates are not exposed to the possiblycorrosive effects of the fluid being measured, and the resultantdifferential pressure transducer is therefore corrosion-proof. This isparticularly useful when the differential pressure transducer isemployed in automotive applications.

Other objects, features, and advantages will become apparent from aconsideration of the following detailed description and from theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view through a transducerillustrating the principles of the present invention;

FIGS. 2 and 3 are cross-sectional views taken as indicated at II--II andIII--III, respectively, of FIG. 1 showing representative metal electrodeconfigurations;

FIG. 4 is an enlarged cross-sectional view of a portion of FIG. 1;

FIG. 5 is an exploded view of an alternative embodiment of theinvention;

FIG. 6 is a view of the assembled transducer of FIG. 5;

FIG. 7 is a cross-sectional view taken along lines VII--VII of FIG. 6;and

FIG. 8 is a partial cross-sectional view illustrating an alternativeembodiment of the invention.

DETAILED DESCRIPTION

Referring more particularly to the drawings, FIG. 1 shows a differentialtransducer assembly including a central relatively thick plate 12 ofquartz, alumina, glass such as Pyrex, or other insulating materialhaving substantially zero mechanical hysteresis. Other ceramic materialsmay also be employed. Mounted on either side of the plate 12 and spacedfrom it are two diaphragms 14 and 16 which may be made of the samematerial as the base plate 12. The diaphragms 14 and 16 may be mountedto the base plate 12 by the peripheral rings of glass frit 18 and 20which are made of finely divided particles of at least two differenttypes of glass, having different melting points so that the spacingbetween the diaphragms 14 and 16, and the base plate 12 may bedetermined by the particle size of the glass frit having the highermelting point. Instead of glass frit, other adhesive material may beemployed to bond and seal the diaphragms to the base plate 12.

The base plate 12 has a central opening 22, through which extends a rod24 which is secured to and mechanically interconnects the two diaphragms14 and 16. The rod 24 may be secured to diaphragms 12 and 14 byadhesives or by fired glass frit. Alternatively, the rod 24 could extendthrough and be sealed to the diaphragms. The rod 24 may be made of thesame material as the diaphragms and the base plate 12, or of a materialhaving the same thermal coefficient of expansion, so that the entireassembly expands and contracts with changes in temperature withoutsignificant loss of accuracy of the sensor.

On the facing surfaces of the central plate 12 and at least one of thediaphragms such as diaphragms 14, are the thin layers of conductivematerial 26 and 28. These may be applied by screening a paste includingthe conductive metal in a silk screen process, and then firing theelements, before they are joined together with the glass frit 18. Thefacing surfaces of the diaphram 16 and the plate 12 may similarly beprovided with conductive surfaces 30 and 32.

Enclosing the differential transducer including central plate 12 anddiaphragms 14 and 16, is a plastic housing including a base member 36,and a cover section 38. The base section 36 is provided with a mountingplate 40 having an openings 42 to receive mounting screws or the like.The base portion 36 is provided with an input conduit 44 for receivingfluid pressure from one source, while the cover plate 38 is providedwith an input conduit 46 for receiving pressure from another source, sothat the output from the pressure transducer is an indication of thedifferential pressure relating to the fluid pressures applied throughports 44 and 46. In order to assure application of the pressure fromport 44 to the diaphragm 16, a rubber gasket 52 is provided which has anL-shaped cross-section so that it bears both on the outer surface ofouter diaphragm 16 and on the inner surface of the housing 36.Similarly, the rubber gasket member 54 seals between the diaphragm 14and the cover portion 38 of the plastic case, to assure that pressurefrom input conduit 46 is applied solely to the diaphragm 14. The metalhousing or rim 62 having inturned edges 64 and 66 serves to hold therubber gasket members 52 and 54 in place in the assembly.

In operation, when fluid pressure from conduit 44 is applied todiaphragm 16, and fluid pressure from conduit 46 is applied to diaphragm14 the two diaphragms will move together either toward conduit 44 ortoward conduit 46 depending on the differential pressure. Of course,with the rod 24 positively interconnecting these two diaphragms, theyare constrained to move together. Further, it may be noted that even ifone or both of the fluids supplied through conduits 44 and 46 arecorrosive, they are only in contact with inert portions such as theinside of the plastic housing 36, 38, and the outer surface of thealumina diaphragms 14 and 16, so that the conductive electrodes may notbe corroded and adversely affected.

Turning now to other figures of the drawings, FIGS. 2 and 3 are views ofthe electrode configuration taken upward and downwardly from the dasheddot lines designated II--II and III--III of FIG. 1. The electrodeconfiguration of FIGS. 2 and 3 may vary considerably from one type ofpressure transducer to another. Essentially it involves the electrodes30 and 32, with the capacitance between these two electrodes increasing,as the diaphragm 16 comes closer to the base plate 12 and similarly, thecapacitance is reduced in value as the diaphragm 16 goes further awayfrom the base plate 12. In FIGS. 2 and 3, the one conductive plate 32 isgrounded, and with reference to FIG. 3, the grounded ring 30-2 enclosesthe active capacitance portion 30-1. The small capacitance element 30-3serves a compensating function, as described in greater detail in U.S.Pat. No. 4,227,419, granted Oct. 14, 1980, and assigned to the assigneeof the present invention.

FIG. 4 is an enlarged cross-sectional view of a portion of FIG. 1, withthe same reference numerals being used in FIG. 4 as were employed inFIG. 1.

Incidentally, it is to be noted that the drawing is not exactly toscale, as the metallized conductive surfaces 26, 28 and 30, 32 wouldactually be very thin, less than one thousandth of an inch thick. Anumber of types of pastes are commercially available which may be silkscreened into the configurations shown in FIGS. 2 and 3, and then firedto form a very thin conductive coating on the underlying material. Inpractice, it is preferred to use a gold coating. Electrical connectionsto the capacitor plates may be made in any convenient manner, and leadsmay conveniently be brought out through openings such as those indicatedat reference numeral 72 in FIG. 2 and at 74 in FIG. 3. The output fromthe variable capacitor is then coupled to a printed circuit having acircuit such as that shown in U.S. Pat. No. 4,398,426, granted Aug. 16,1983, and assigned to the assignee of the present invention. The printedcircuit may, for example, be mounted in the base of the housing shown inthe present drawings, or may be mounted separately. Concerning anotherminor matter, the two diaphragms 14 and 16 are preferably sealedperipherally to the central plate 12 to avoid contamination of theinterior of the transducer; however, as long as the two input conduits44 and 46 are hermetically sealed to respective periphery of thediaphragms 16 and 14, the space within the transducer need not besealed. If desired, one of the two pairs of plates 26, 28 or 30, 32 maybe eliminated. Alternatively, the increase in capacitance of one set ofplates and the concurrent decrease in capacitance of the other set ofplates may be processed separately to indicate the differentialpressure, and the results averaged.

As mentioned above, FIGS. 5, 6 and 7 relate to an alternative embodimentof the invention.

FIG. 5 is an exploded view of the differential pressure transducer andincludes two housing members 82 and 84. These housing members are heldtogether with six screws 86, which pass through openings in the housingsection 82, are secured into tapped holes in the housing section 84, andare held in place by lock washers 88. The rubber gasket member 90 ismounted in a groove 92 which extends around the central opening in thehousing members 84 and 82, and serves to seal the two housing partstogether when they are pulled together by the screws 86.

A central transducer member 94 provided with outer diaphragms 96 and 98,is mounted between a pair of cushioning gasket members 100 and 102 whichalso serve to hold the two metal shielding members 104 and 106 in theirproper positions on either side of the transducer 94. A small printedcircuit board 108 provides the electronic circuitry for processing thevariable capacitance signals from the transducer 94 and providing avoltage output signal which is proportional to the changes in pressure.The printed circuit board 108 serves to implement the circuitrydescribed in the U.S. patents assigned to the assignee of this patentapplication, which were cited hereinabove. The printed circuit board 108is mounted on the insulated plate 110, which may be either ceramic ormolded plastic. Incidentally, as an alternative configuration, thecentral member of the transducer 94 may be extended in a shapesubstantially similar to the outline of the opening in the housing 84and similar to the configuration of the gasket but somewhat smaller, andthe printed circuit board 108 may be mounted on this extended ceramicbase member. As a further alternative, the alumina plate of the centralmember of the transducer 94 may form the substrate for the printedcircuit elements which are shown in FIGS. 5 and 7 on the separateprinted circuit board 108.

The output signals from the printed circuit board 108 are coupled by theconnector 114 to the three output leads 116. The rubber gasket 118serves to seal the connector assembly 114 into the housing member 82.The connector 114 may be secured to the housing 82 by staking, bygluing, by ultrasonic welding, or by any other suitable technique.

As best shown in FIG. 7, the housing members 82 and 84 have outwardlyextending coupling tubes 122 and 124 respectively, to which the fluidpressures to be measured are applied. The pressure from the input 122 isapplied to diaphragm 96, while the pressure applied through input port124 is applied to the diagraphm 98. With the two diaphragms beingphysically interconnected by the linkage 126, the deflection of thediaphragms is proportional to the difference in pressure between the twofluids which are being measured. Incidentally, the linkage element 126is preferably made of the same material as the diaphragms 96 and 98, andthe central base member 94 of the transducer, or it may be made of adifferent material having substantially the same thermal coefficient ofexpansion. Further, the element 26 may be secured to the diaphragms byglass frit, in the same manner as the diaphragms are secured to thecentral base member.

In the arrangement shown in FIGS. 5 through 7 of the drawings, the twohousing members 82 and 84 are plastic. As an alternative, these twoouter members could be formed of metal, such as aluminum. With thehousing members being formed of plastic, the inner surfaces 132 of thehousing members are preferably coated with conductive material forincreased electrical shielding. In addition, as mentioned above, theshielding members 104 and 106 are provided to further shield thevariable capacitance plates which are provided between one of thediaphragms 96 or 98, and the central base member of the transducer 94.Of course, when aluminum housing members are employed, no supplementalconductive coating would be required on the inside surfaces of thesehousing members.

It is also noted in passing that the spacing of the diaphragms 96 and 98relative to the central base member is so small that the spacing doesnot show in the cross-sectional view of FIG. 7. However, it is of theorder of magnitude discussed above in connection with the firstembodiment of the invention.

FIG. 8 is a partial cross-sectional view illustrating an alternativeembodiment of the invention in which the two diaphragms 96-1 and 98-1are intercoupled by silicone oil. The silicone oil 132 fills the spacesbetween the diaphragm 96-1 and the central fixed plate 96-1, and betweenthe diaphragm 98-1 and plate 94-1, as well as the openings 134, 136, 138extending through the central plate 94-1. In order to minimizetemperature effects, the volume of silicone oil is kept to a minimum byproviding a plurality of small diameter holes 134, 136, 138. In thisregard, the hole 136 may be centrally located, and a series of spacedholes including holes 134 and 136 may be evenly spaced around thecentral hole. With the spacing between the diaphragms and the centralplate normally being in the order of one to a few thousandths of aninch, and the holes being of relatively small diameter, the effects oftemperature changes resulting in a small differential expansion of thefluid, will be minimal.

In conclusion, it is to be understood that the foregoing detaileddescription and the accompanying drawings merely relate to oneillustrative embodiment of the invention. Variations from the specificdesign shown hereinabove are possible within the scope of the presentinvention. Thus, by way of example and not of limitation, the electrodeconfiguration may be different from that shown in FIGS. 2 and 3, andcould be those, for example, shown in other patents cited hereinabove.Also, concerning the housing and the rubber sealing rings, otherarrangements could be employed to accomplish the same functions; forexample, O-rings may be employed instead of the rubber gasket ringshaving an L-shaped configuration. In addition, the diaphragms, thecentral base plate, and the connecting rod may be formed of any suitableinsulating material having substantially zero mechanical hysteresis.Various materials which may be employed include alumina, which ispreferred, quartz, Pyrex glass, and other ceramics falling within thecategory mentioned hereinabove. Accordingly, the present invention isnot limited to the construction precisely as described in detailhereinabove and as shown in the drawings.

What is claimed is:
 1. A corrosion proof pressure transducer assemblyfor measuring the pressure difference between two fluids, comprising:acentral circular insulating base member having a central opening and twospaced flat sides; first and second thin circular, flat insulatingdiaphragms having substantially zero mechanical hysteresis, and eachhaving a diameter substantially equal to that of said base member; meansfor mounting said diaphragms, one on each side of said base member andspaced apart from said base member by between 0.0005 and 0.010 inch; acoupling member having substantially the same coefficient of thermalexpansion as said insulating base member, mechanically interconnectingthe centers of said pair of diaphragms and extending through the centralopening in said base member; facing conductive means in the form ofconductive surfaces on the inner surface of one of at least one of saidceramic diaphragms and the adjacent surface of said base member, forforming a variable capacitor having a capacitance which varies as thediaphragms flex; housing means having first and second input ports forapplying fluid pressures from two different sources directly to theouter surfaces of said first and second diaphragms, respectively; andresilient means mounted near the peripheries of said first and seconddiaphragms directing and limiting the fluid path from each of said inputports to the associated diaphragm; whereby the capacitance between saidtwo conductive surfaces is a function of the difference in the fluidpressures applied to said two diaphragms, and said conductive surfacesare isolated from any possible corrosion from the fluids being measured.2. A differential pressure transducer assembly as defined in claim 1wherein said housing means is a plastic housing having first and secondinput conduits; and resilient gasket means for directing fluid pressurefrom said first and second conduits to the surfaces of the respectivefirst and second diaphragms.
 3. A differential pressure transducerassembly as defined in claim 1 wherein said coupling member, saiddiaphragms and said base member are all formed of the same type ofmaterial.
 4. A differential pressure transducer assembly as defined inclaim 1 wherein each of said diaphragms is between 0.010 inch and 0.10inch in thickness.
 5. A differential pressure transducer assembly asdefined in claim 1 when said two diaphragms and said base member areeach between one and two inches in diameter.
 6. A differential pressuretransducer assembly as defined in claim 1 wherein said diaphragms andsaid base member are formed of alumina.
 7. A differential pressuretransducer assembly as defined in claim 1 wherein said diaphragms arespaced from and peripherally sealed to said base member by glass frit.8. A differential pressure transducer assembly as defined in claim 1including conductive surfaces on the inner surfaces of said first andsecond diaphragms and on the facing surfaces of said base member to formtwo variable capacitors.
 9. A corrosion-proof pressure transducerassembly for measuring the pressure difference between two fluids,comprising:a central insulating base member having at least one openingand two spaced flat sides, said base member having flat sides extendingall of the way across both sides thereof; first and second thin flatinsulating diaphragms having substantially zero mechanical hysteresis;means for mounting said diaphragms, one on each side of said base memberand spaced apart from said base member by between 0.0005 and 0.010 inch,and peripherally secured to said basemember; means for intercouplingsaid pair of diaphragms and extending through the opening in said basemember; means responsive to the flexing of at least one of saiddiaphragms for producing an electrical output signal proportional to thedifferential pressure; housing means having first and second input portsfor applying fluid pressures from two different sources directly to theouter surfaces of said first and second diaphragms, respectively;resilient peripheral sealing means for sealing against fluid pressureleakage between said housing and the outer edges of said diaphragms; andsaid assembly including conductive means for shielding said transducer;whereby the electrical output signal is a function of the difference inthe fluid pressures applied to said two diaphragms.
 10. Acorrosion-proof pressure transducer assembly for measuring the pressuredifference between two fluids, comprising:a central circular insulatingbase member having a central opening and two spaced flat sides; firstand second thin circular, flat insulating diaphragms havingsubstantially zero mechanical hysteresis, and each having a diametersubstantially equal to that of said base member; means for mounting saiddiaphragms, one on each side of said base member and spaced apart fromsaid base member by between 0.005 and 0.010 inch; a coupling memberhaving substantially the same coefficient of thermal expansion as saidinsulating base member, mechanically interconnecting the centers of saidpair of diaphragms and extending through the central opening in saidbase member; facing conductive means in the form of conductive surfaceson the inner surface of one of at least one of said diaphragms and theadjacent surface of said base member, for forming a variable capacitorhaving a capacitance which varies as the diaphragms flex; housing meanshaving first and second input ports for applying fluid pressures fromtwo different sources to the outer surfaces of said first and seconddiaphragms, respectively; said transducer assembly including an innermetal housing extending around at least the periphery of said diaphragmsand base member; whereby the capacitance between said two conductivesurfaces is a function of the difference in the fluid pressures appliedto said two diaphragms, and said conductive surfaces are isolated fromany possible corrosion by the fluids being measured.
 11. Acorrosion-proof pressure transducer assembly for measuring the pressuredifference between two fluids, comprising:a central insulating basemember having at least one opening and two spaced flat sides; first andsecond thin flat insulating diaphragms having substantially zeromechanical hysteresis; means for mounting said diaphragms, one on eachside of said base member and spaced apart from said base member bybetween 0.0005 and 0.010 inch, and peripherally secured to said basemember; means for intercoupling said pair of diaphragms and extendingthrough the opening in said base member; facing conductive means in theform of conductive surfaces on the inner surface of one of at least oneof said diaphrams and the adjacent surface of said base member, forforming a variable capacitor having a capacitance which varies as thediaphragms flex; enclosing means having first and second input ports forapplying fluid pressures from two different sources to the outersurfaces of said first and second diaphragms, respectively; andresilient gasket means for directing fluid pressure from said first andsecond input ports to the surfaces of the respective first and seconddiaphragms; whereby the capacitance between said two conductive surfacesis a function of the difference in the fluid pressures applied to saidtwo diaphragms, and said conductive surfaces are isolated from anypossible corrosion by the fluids being measured.
 12. A differentialpressure transducer as defined in claim 11 including a plastic housinghaving first and second input conduits, and resilient gasket means fordirecting fluid pressure from said first and second conduits to thesurfaces of the respective first and second diaphragms.
 13. Adifferential pressure transducer assembly as defined in claim 12 whereinsaid assembly includes metal housing means extending around at least theperiphery of said diaphragm and base member assembly for holding saidgasket means in place.
 14. A differential pressure transducer assemblyas defined in claim 11 wherein said coupling member, said diaphragms andsaid base member are all formed of the same type of material.
 15. Adifferential pressure transducer assembly as defined in claim 11 whereinat least one of said diaphragms is between 0.010 inch and 0.10 inch inthickness.
 16. A differential pressure transducer assembly as defined inclaim 11 wherein said diaphragms and said base member are formed ofalumina.
 17. A differential pressure transducer assembly as defined inclaim 11 wherein said diaphragms are spaced from and peripherally sealedto said base member by glass frit.
 18. A corrosion-proof pressuretransducer system for measuring the pressure difference between twofluids, as defined in claim 11 further including electronic circuitrymeans mounted within said enclosing means and coupled to said variablecapacitor means, for providing an output electrical signal which varieswith changes in differential pressure.
 19. A corrosion-proof pressuretransducer system for measuring the pressure difference between twofluids, as defined in claim 11 further including means for shieldingsaid variable capacitor from stray electrical signals.
 20. Acorrosion-proof pressure transducer system for measuring the pressuredifference between two fluids, as defined in claim 19 wherein saidshielding means includes apertured metal shields extending over each ofsaid diaphragms.
 21. A corrosion-proof pressure transducer system formeasuring the pressure difference between two fluids, as defined inclaim 19 wherein said shielding means includes conductive materialextending generally co-extensively with said enclosing means.
 22. Acorrosion-proof pressure transducer system as defined in claim 11,wherein said intercoupling means is a fluid filling the space betweensaid diaphragms and said base member and extending through said hole.23. A corrosion-proof pressure transducer system as defined in claim 22wherein said base member has a plurality of holes therethrough forintercoupling the spaces between said diaphragms and said base member.24. A corrosion-proof pressure transducer assembly for measuring thepressure difference between two fluids, comprising:a central circularinsulating base member having a central opening and two spaced flatsides; first and second thin circular, flat insulating diaphragms havingsubstantially zero mechanical hysteresis, and each having a diametersubstantially equal to that of said base member; means for mounting saiddiaphragms, one on each side of said base member and spaced apart fromsaid base member by between 0.0005 and 0.010 inch; a coupling memberhaving substantially the same coefficient of thermal expansion as saidinsulating base member, mechanically interconnecting the centers of saidpair of diaphragms and extending through the central opening in saidbase member; facing conductive means in the form of conductive surfaceson the inner surface of one of at least one of said diaphragms and theadjacent surface of said base member, for forming a variable capacitorhaving a capacitance which varies as the diaphragms flex; housing meanshaving first and second input ports for applying fluid pressures fromtwo different sources to the outer surfaces of said first and seconddiaphragms, respectively; and first and second resilient rings mountednear the peripheries of said first and second diaphragms directing andlimiting the fluid path from each said input ports to the associateddiaphragm; whereby the capacitance between said two conductive surfacesis a function of the difference in the fluid pressures applied to saidtwo diaphragms, and said conductive surfaces are isolated from anypossible corrosion by the fluids being measured.
 25. A corrosion-proofpressure transducer assembly as defined in claim 24 wherein said centralbase member is entirely flat across its entire surface, and at least oneof said diaphragms is spaced from and sealed to said base member byglass frit.